The present invention relates to a cavity resonator system for measuring electro-magnetic (EM) properties of the contents of a pipe portion, and in particular that can provide compensation for changes in environmental conditions such as temperature and pressure.
Cavity resonators are widely used to measure the dielectric and other EM properties of the contents of a pipe, typically a fluid flowing through the pipe. It is possible to continuously determine, for example, the volume fractions of mineral oil and water emerging from a well, which may be at the surface or subsea. Similarly, the cavity resonator may be used inline with a flow of drilling fluid to measure its water content and salinity. Likewise, the cavity resonator may be deployed inside the Christmas tree at the well-head, or downhole inside a well, to provide in-situ measurements of water cut, water ‘hold-up’ and salinity.
The cavity is typically formed in an outer conductive casing of metal with the cavity around the pipe completely filled with an insulator material that may be a solid for applications where it is desired to minimize the deformation of the casing under pressure. However, the EM properties of the insulator material change with the environmental conditions such as temperature and pressure, and this can cause significant errors that is variation in the measured response that are not caused by the contents of the pipe under measurement.
For example, one parameter of the resonant EM field in the cavity is a resonance frequency of the cavity. Such a resonance frequency is particularly sensitive to changes in the dielectric properties of the contents of the pipe. However, the resonance frequency is also affected by changes in the cavity dimensions and changes in the permittivity of the insulator material that fills the cavity outside the pipe. Accordingly, it is difficult to determine the dielectric properties of the contents of the pipe from the measured resonance frequency, due to the dependence on the environmental conditions.
One solution would be to attempt to sense the environmental conditions and calibrate the measured parameters on that basis. However, such sensing is impractical in many applications, particularly extreme ones such as are encountered in the petrochemical extraction industry. Furthermore, calibration is difficult to perform accurately.
It would be desirable to tackle errors of this nature in a cavity resonator system used to measure the EM properties of the contents of a pipe portion.